Continuous casting apparatus with a molten metal level control



CONTINUOUS CASTING APPARATUS WITH A MOLTEN METAL LEVEL CONTROL FiledFeb. '7, 1968 G. VISCHULIS July 7, 1970 5 Sheets-Sheet 1 find fnuenfvrxwQwUNVW .wAA/

16 Gem 9 Vase/Liz's Diff/((61 144 PV B P Q G. VlSCHULlS 3,519,060

CONTINUOUS CASTING APPARATUS WITH A MOLTEN METAL LEVEL CONTROL July 7,1970 5 SheetsSheet 5 Filed Feb. '7, 1968 m itqmL T mil wumSBmS T 4 HQQFkDnZS 3 9m E :55

AUQL 77(1) 0 -m mm. i wzEmwkm QM umo i8 4 Qmmmruh-im 3 i3 Q GIDX- EZ mLslhao on. XoQ u COM 00 08 00m 02 O Q2: 3: 39 com 03 e2. 03 wfitoumm DFiled Feb. 7, 1968 CONTINUOUS CASTING APPARATUS WITH A MOLTEN METALLEVEL CONTROL 5 Sheets-Sheet k An n n n 70 'A is g Q STEERING {72 FSYMETR. r rq B A YRUN A COIL. 'L Wfifll/lMRI1lb- W u I A UPPER "LOCKOUT"swm m COIL 50 A 114 I SWITCH 1 105 W 10 Z12 f WAVE 2 w v COIL MIXERDETECTOR s V W W osc. DETEcToRY GATE 01 61 63/ i 65 WIN WWII! \con. WAVESWITCH 60 1U6 A COIL A BEAT A'EC' HMUUHUUM- (LOWER) WWWWWUWW osc. AMP. I

y 7, 1970 G. VISCHULIS 3,519,060

CONTINUOUS CASTING APPARATUS WITH A MOLTEN METAL LEVEL CONTROL FiledFeb. '7, 1968 Sheets-Sheet 5 f SAMPLE PULS E GATE WIDTH u DETECTOR U L"RESET" swwcu /1/30 33 sgrzm E PULSE T WIDT L DETECTORL 81 T A "5555;AVERAGTNG A LE SWITCH 130 5 #3 139 A RESET OUTPUT D.C.LEVEL J26" U U U0| FFER. COM PEN.

T'MER B AMP. AMP. E M-i 87 DELAY m 124 TIMER M|- ILRUN 120 T SAMPLEINTEGRATE A 2:? B DIFFER.

UU UU Ulr' AMP. M 1

RUN 22 SAMPLE J A AUX. v DERWATIVE 37 R-s F-F r n n D|FFER.

A AMP. H] H 1 -51 DUAL POSITION FEEDBACK 1 MM 6 I su I 1 L91 Z4] 90-OUTPUT 5Z A M P. SERVO SERVO POWER AM P.

United States Patent "ice 3,519,060 CONTINUOUS CASTING APPARATUS WITH AMOLTEN METAL LEVEL CONTROL George Vischulis, Hickory Hills, 11].,assignor to Interlake Steel Corporation, Chicago, [1]., a corporation ofNew York Filed Feb. 7, 1968, Ser. No. 703,750 Int. Cl. B22d 11/10; B22c19/04 U.S. Cl. 164155 Claims ABSTRACT OF THE DISCLOSURE A pair of coilsencircling the mold of a continuous casting machine are connected to anoscillator to produce oscillations having a frequency dependent on thelevel of molten metal. The oscillations are compared by a differentialcircuit which determines the difference in the oscillatory frequency ofthe coils, and eliminates changes caused by common mode temperatureeffects, to generate a level signal which controls the level of moltenmetal within the mold.

This invention relates to level detector apparatus for indicating andcontrolling the level of a conductor confined within a boundary.

The invention is particularly adapted for sensing the level of liquid ormolten metal within a metal processing machine, and using the levelindication to maintain the molten metal at a predetermined level. Forexample, in a continuous casting machine, molten metal is poured into anelongated mold through which it passes and cooled to produce acontinuous cast output strip or billet. The mold is formed fromvibrating sections having both transverse and longitudinal reciprocatingmovement following a harmonic or elliptical path, such movement causingthe mold sections to repeatedly contact and propagate the casting in aforward stroke. An example of a continuous casting machine is shown inU.S. Pat. 3,075,264 to Wognum, to which reference should be made forbackground information. In order to control the casting process, it isnecessary to maintain the molten metal at a predetermined level in themold.

Attempts to use a change in molten metal level to produce an inductiveor other related type change in a coil which can then be converted intoa level signal have been unsuccessful due to a number of factors. Animportant limiting factor is the intense heat generated during thecasting process, which is conveyed to the surrounding structure andcauses extreme temperature ariations as the level of molten metal varieswithin the mold. Such temperature variations may produce a change ofinductance in a coil which will exceed the inductance change caused bythe variation in the level of molten metal adjacent the coil.

Another important factor which limits the type of level detector usablewith a continuous casting machine is the presence of large amounts ofmetal in the machine itself, which acts as a shorted secondary windingand desensitizes a coil, thus masking the change in inductance otherwisecaused by the variation in metal adjacent the coil.

For these reasons, continuous casting machines and the like have usedgamma ray detectors or thermal cables located at spaced locations withinthe mold wall of the casting machine to sense the level of molten metalwithin the mold. While gamma ray detectors are responsive to a percentchange in transmittance of radiated gamma rays to determine the level ofthe metal, they are ineffective for certain metals, such as aluminum,which have a high gamma ray transmittance, and the signal re- 3,519,060Patented July 7, 1970 spouse is low Where the cast metal scanned issmall relative to the entire metal scanned. Gamma ray detectors alsopresent a safety hazard in terms of radiation. Thermal cables mountedwithin the mold wall have problems in that the mold walls in acontinuous casting machine must oscillate away from the cast metal forapproximately one-half of the time. Both systems are of relatively slowresponse time, and limit the speed at which a continuous casting machinemay be operated.

A satisfactory level detector system requires a minimum amount ofequipment located at the continuous casting machine itself, and suchequipment at the machine should be of a type which can be easily addedto existing machinery without requiring substantial modifications. Noneof the prior art level detectors used with continuous casting machinesmeets all of these diverse requirements.

In accordance with the present invention, novel apparatus is disclosedfor determining the level of a conductor confined within a boundary,which apparatus is especially adapted for determining and controllingthe level of molten metal in a continuous casting machine. Plural coilshaving a particular type of change in an electrical characteristic for achange in metal level are sensed in a differential manner to produce alevel signal independent of fluctuating temperatures, other extraneousinfluences affecting inductance and the presence of large bodies ofmetal in the vicinity of the sensing system.

One object of this invention is the provision of an improved leveldetector for indicating and/ or controlling the level of a conductorconfined within a boundary.

Another object of this invention is the provision of a level detectorand control for a continuous casting machine, which uses an inductiveand an AC resistance effect to determine the level of molten metalwithin the mold of the casting machine.

One feature of this invention is the provision of a level detector for aconfined conductor, having a plurality of winding means located adjacentthe boundry of a conductor, and a differential circuit coupled to theplurality of winding means for sensing a level signal which isindependent of a common mode change in inductance of the winding meanscaused by temperature variations or other factors not directly relatedto the actual level of the conductor.

Another feature of this invention is the provision of a level detectorfor a continuous casting machine, which connects a plurality of coilssurrounding the machine mold to an oscillator to produce self-resonantoscillations having a frequency dependent upon the value of inductanceand AC resistance of the coils.

Yet another feature of this invention is the provision of a leveldetector which alternately connects a pair of coils to an oscillator toproduce trains of self-resonant oscillations, the phase differencebetween the trains being measured by a differential circuit whichincludes means for compensating for common mode changes in the frequencydetermining characteristics of both coils.

Futher features and advantages of the invention will be apparent fromthe following description, and from the drawings, in which:

FIG. 1 is a top plan view of a continuous casting machine and a portionof the level detector apparatus;

FIG. 2 shows a front elevation of a continuous casting machine as viewedalong lines 2-2 of FIG. 1, and more particularly shows the top portionof a continuous casting machine and a portion of the level detectorapparatus;

FIG. 3 is a block diagram of the level detector apparatus for detectingand controlling the level of metal in the continuous casting machine ofFIGS. 1 and 2;

FIG. 4 is a curve of oscillatory output frequency versus metal levelwhen the level determining coils of FIGS. 1

3 and 2 are connected in circuit with the oscillator of FIG. 3;

FIGS. 5A and 5B are a single schematic diagram of the circuit of FIG. 3,in which the broken lines across the right-hand side of FIG. 5A and theleft-hand side of FIG. 5B are placed together to show the continuationof the lines across the drawings; and

FIGS. 6A6T are diagrams illustrating the time relationship between thesignals generated by the circuit of FIG. 5.

While an illustrative embodiment of the invention is shown in thedrawings and will be described in detail herein, the invention issusceptible of embodiment in many different forms and it should beunderstood that the present disclosure is to be considered as anexemplification of the principles of the invention and is not intendedto limit the invention to the embodiment illustrated. The scope of theinvention will be pointed out in the appended claims.

GENERAL OPERATION OF CONTINUOUS CASTING MACHINE Turning now to FIGS. 1and 2, a continuous casting machine 10 is generally illustrated forgradually solidifying a mass of molten metal 11 poured from a tundish 12into a mold 14 for the machine. Mold 14 is composed of four moldsections 16 which are generally rectangularly block-shaped, dependingupon the shape desired for the cross-sectional area of the final castproduct. Each mold section 16 is held by a mold retainer 18 which isvibrated through an elliptical orbit by a conventional motor 19. To sealthe line between adjacent mold sections 16, a stationary corner strip 20is held in place by a corner strip retainer 21 which is fixedly attachedwith respect to the base of machine 10.

As is known in the art of continuous casting, as exemplified by U.S.Pat. 3,075,264, the particular type of orbital motion produced by motor19 and associated linkages is such that opposite mold sections 16 arebrought into contact with the metal 11 billet or strip being cast withinmachine 10 while those mold sections are moving toward each other anddownwardly. Before the bottom of the elliptical stroke, the moldsections move away from each other and the metal billet, and thereafterrise upwardly on a return stroke in a direction opposite to thedirection of travel of the metal through the mold. During this returnstroke, the other opposite pair of mold sections 16 move toward eachother and downwardly, continuing to urge the metal downwardly throughmold 14.

The stroke is ordinarily maintained through a fixed distance or path,while the frequency or rate of completing a single elliptical path isvaried to change the rate of metal flow through the machine. Forexample, each of the mold sections may make from 1,000 to 2,500revolutions per minute, while following the path of a 10 to 1 ellipse.The orbital amplitude by which the mold sections withdraw from the metalstrip is very small, and is typically on the order of a few thousandthsof an inch. To progressively cool the molten metal 11 as it passesthrough mold 14, in order that it may gradually solidify and emergecontinuously belowthe mold sections as a solidified cast billet orstrip, mold retainers 18 and mold strips 16 are of hollow constructionto permit water or other cooling fluid to flow up through the retainer18 and downwardly along the inner surface of mold section 16 in order tocool the molten metal adjacent the surface of the mold. For thispurpose, a continuous aperture or opening 25 is formed through theelongated portions of the mold 14 in order to permit water flow 26 topass therethrough and cool the machine.

LEVEL SENSING COILS In order to control the quality of the cast billet,and other factors including preventing the metal pour from overflowingfrom the top of mold 14, it is important to determine the point 30 atwhich the thin molten metal pour spout flows outwardly and firstcontacts the surface wall of the mold sections 16. The level point 30will be referred to hereinafter as the level of the metal within themold, although it will of course be apparent that some metal, in theform of the thin pour spout from the tundish 12, extends above theso-called level of metal within the mold.

A level sensor, in the form of coil means 32 located adjacent mold 14,provides an electrical condition which changes in accordance withchanges in the level 30' of metal 11 within the mold. This change inelectrical condition is possible because mold 14 is discontinuous orsegmented, as will be explained in detail later. Means 32 comprises afirst coil U and a second coil L which may be split into an uppersection L located above coil U and a lower section I." located belowcoil U, for reasons to be explained hereinafter. The net inductiveeffect of the pair of split coil sections L and L" is effectively thesame as a single coil L located slightly below coil U. The coils U and Lare wound on a coil form or bobbin 35 which encircles the mold 14, andwhich is sealed by an encircling insulated form 36.

A coil shield 3'8 surrounds coil forms 35, 36 and performs the dualfunction of electrically isolating the coils U and L from the elfects ofextraneous metal bodies located near the coils, and forming a splashshield to prevent molten metal 11 from contacting the coils or coilforms. Shield 38, preferably formed of conductive material such ascopper or stainless steel, is especially effective as a barrier whichisolates coils U and L from the effect of tundish 12 and the varyinglevel of molten metal 11 within the tundish. Coil means 32 is rigidlymounted by fastening means 40 to a base 41 which is fixed with respectto machine 10 and tundish 12.

As the level 30 of molten metal rises towards the top of mold 14, thevolume of the slug of molten conductor within the encircling coils U andL increases. It has been found that this causes the inductance of eachcoil to decrease and the AC resistance of each coil to increase due toincreased eddy current losses occurring within the center slug or massof molten conductor. The eddy current losses within the molten conductoroccur because mold 14 is not formed from a continuous annular part, butrather from segmented mold sections. More particularly, if mold 14, asviewed in FIG. 1, was formed from a continuous encircling or annularmold form (instead of separate mold sections 16), the magnetic fluxgenerated when the coils U and L are connected to an oscillator wouldinduce a current which would entirely encircle the molten conductor.This induced current would in turn generate a flux which would cancelthe flux in the molten conductor, preventing circulating eddy currentsfrom being induced in the molten conductor.

However, by providing the combination of a mold with a discontinuity tothe flow of induced currents and a magnetic flux field generating means,a circulating current is induced which does not completely encircle themolten conductor. As a result, circulating or eddy currents are inducedin the molten conductor, creating a power loss which is reflected backto the magnetic field generating means as a change in the inductance andthe AJC resistance of the coils. By detecting either or both theinductance and AC resistance characteristic of the coils, the amount ofeddy current loss, which is proportional to the level of metal, can bedetermined.

The term discontinuous in this context refers to a nonhomogeneousbarrier which substantially prevents an induced circulating current fromflowing through the barrier. Entirely separate mold sections 16, FIG. 1,are only exemplary of one manner of forming a boundary having adiscontinuity to the flow of an induced current. In the illustratedembodiment, the formation of a metal oxide layer on each mold section isbelieved to aid to providing the discontinuity to the flow ofcirculating currents.

The circulating current losses in the molten conductor change with thelevel of the conductor, and thus provide a means of detecting theconductor level. Since the circulating or eddy currents losses arereflected back and change both the inductance and the AC resistancecharacteristics of the coils U and L, either or both characteristics canbe monitored to indicate metal level. The circuit of FIG. 3 uses thesecharacteristics of the coils in order to provide an indication of thelevel 30 of the molten metal within mold 14.

GENERAL OPERATION OF LEVEL DETECTOR AND CONTROL Turning now to FIG. 3,each coil U and L is connectable with an oscillator 50 in order toproduce natural oscillations having a frequency dependent upon the valueof the inductance and the AC resistance of the coil. The frequency ofoscillation, as is well known, is inversely proportional to theinductance and directly proportional to the AC resistance of a coil. Foran increase in metal level, the inductance of the illustrated coilsdecrease while the AC resistances of the coils increase. Since bothelectrical effects contribute to an increase in frequency ofoscillations, the oscillatory frequency can be used as a directindication of the metal level. While the preferred circuit of FIG. 3 isresponsive to the frequency of oscillations, affected both by theinductance and the AC resistance characteristics of the coils, a circuitcould of course be used which was responsive to one of these electricalcharacteristics by itself.

The relative difference in the frequency of oscillations produced byoscillator 50' when connected to coils U and L, being representative ofthe difference in inductance and AC resistance between the coils, isused to generate an output signal on line 5-1 which drives a servo motor52 for returning the level of molten metal to a predetermined desiredvalue. Servo motor 52 may form a part of any conventional servomechanismfor controlling the level of molten metal. By way of example, motor 52could be connected by conventional circuitry to control the revolutionsper minute of motor 19 of FIG. 2, which drives the mold sections 16 inthe continuous caster. Such operation, affecting the rate at which themold sections 16 complete a single cycle of elliptical motion, variesthe rate of travel of the molten metal. By thus controlling the rate atwhich metal is removed from mold 14, the level of the molten metalwithin the mold is controlled. Alternatively, servo motor 52 could beconnected to a valve (not illustrated) on tundish 12 in order to controlthe amount of molten metal flowing into mold 14, and hence alsocontrolling level 30.

As the level 30 of molten metal varies within mold 14, the resultingchange in the electrical characteristics of the coils causes thefrequency output from oscillator to vary, as can be seen with referenceto FIG. 4. Lower coil L is comprised of a greater number of turns thanupper coil U, and hence has a greater range of frequency deviation, whenconnected to oscillator 50, for a given change in metal level. The coilsU and L are de-tuned or otherwise so chosen so that the frequency curveof one of them is shifted relative to the other in order to produce across-over point at which the inductance of each coil is equal. Forlevels of metal above cross-over point 55, it is noted that theinductance of one of the coils (in this case coil L) decreases at agreater rate than the other coil, hence producing a higher frequency ofoscillations, while for metal levels below cross-over point 55, thecurves change positions, and the one coil (namely coil L) now increasesin inductance at a greater rate than the other coil, and hence causes alower frequency of oscillations.

By de-tuning the pair of coils in this manner, a characteristic curve isproduced which can uniquely determine metal level. More particularly,the magnitude of the difference in the frequency of oscillationsoccurring when the two coils are connected to an oscillator is directlyproportional to the distance variation of the metal level from areference value. Furthermore, the frequencies generated by a particularcoil, in this case coil L, are of higher value for metal levels abovethe reference level, and of lower frequency relationship than the othercoil for metal levels below the reference level, and thus provide aunique indication of the direction in which the level has varied. Whenconnected as a servomechanism for returning metal level to a desiredlevel, it is preferred that cross-over point 55 be chosen to occur atthe level at which the metal is to be maintained, so that a zero voltsignal is produced when the desired level of metal is maintained.

Returning to the circuit of FIG. 3, a single time shared oscillator 50is provided for both coils U and L. Such a circuit is preferred in orderto eliminate different temperature drift effects which would occur withseparate oscillators for each coil. An electronic switch 60 alternatelyconnects coil U and coil L to oscillator 50, in order to produce at anoutput line 61 a train of oscillatory signals with continuouslyalternate portions of the train being attributable to either coil U orcoil L. The train of oscillations is heterodyncd in a mixer-detector 63with continuous oscillations from a beat frequency oscillator 64 inorder to produce a different or beat frequency on an output line 65 ofdetector 63.

Since oscillator 50 is alternately switched between the two coils, theparticular portions of the beat frequency on line 65 which isattributable to each coil must be determined. A central clock 70continuously produces timing pulses which are used for referencepurposes in the circuit. Clock 70 drives a steering flip-flop (PF) 72,to alternately enable one of a pair of output lines connected withelectronic switch 60. The enabled line triggers the electronic switchassociated therewith to connect one of the coils with oscillator 50. Thesteering flip-flop 72 also produces output signals which are connectedwith remaining portions of the circuitry in order to synchronize theoperation of various components with the connection of coil U or L tothe oscillator 50.

The train of oscillations from detector 63 are coupled in parallel to asample gate 80, responsive to portions of the train of oscillationsattributable to coil U, and a sample gate 81, responsive to theremaining portions of the train of oscillations attributable to coil L.Steering flip-flop 72 actuates or enables gate at the same time thatelectronic switch 60 connects coil U to oscillator 50. Similarlly, whenelectronic switch 60 connects coil L to oscillator 50, sample gate 81 isenabled.

While the relative inductance and AC resistance difference between coilsU and L could be determined in several manners, in the illustratedembodiment, the sample gates 80 and 81, when enabled steering flip-flop72, pass a portion of the third cycle of oscillation to pulse widthdetector 83 for that sample gate. In order to open the sample gates 80and 81 for the third cycle of oscillation, and thereafter close thesample gates prior to the fourth cycle of oscillation, a timing circuit85 coupled between clock 70 and the sample gates provides a plurality oftiming pulses, to be described in detail later.

As the frequency output from oscillator 50 increases or decreases fromthe nominal frequency output occurring at cross-over point 55 of FIG. 4,the train of oscillations appears to be expanded or compressed, therebyvarying the width of the portion of the third cycle of oscillation whichis passed by sample gates 80, 81 to detectors 83. Each detector 83produces an analog output signal which is directly proportional to thewidth of the pulse coupled thereto.

The output of both of the pulse width detectors 83 is coupled to adifferential amplifier 87 to produce a single output signalrepresentative of the difference between the two input signals. If theoutputs of each of the pulse width detectors 83 are equal, it indicatesthat both coil U and coil L are generating the same frequency ofoscillation, which occurs only at cross-over point 55 of FIG. 4, andhence a zero voltage is produced by differential amplifier 87. Shouldthe analog signal from one of the pulse width detectors 83 exceed theother, differential amplifier 87 will produce a corresponding outputsignal of one polarity, representative of the difference therebetween.Conversely, should the outputs from the pulse width detectors 83 vary inthe opposite direction, an opposite polarity signal representative ofthe difference therebetween will be generated.

The output signal from differential amplifier 87 is directlyproportional to the difference between the oscillatory frequencies forthe L and U coils. More particularly, the polarity of the output signalindicates whether the level is rising or falling from the desired orreference level, whereas the magnitude of the output signal indicatesthe amount or distance the metal level has varied from the desiredlevel.

To control the level of the molten metal, line 51 is connected to aservomechanism which includes servo motor 52 for controlling either therate of adding molten metal to the continuous casting machine, or therate of withdrawing metal from the continuous casting machine, aspreviously described. The servomechanism includes a summing outputamplifier 90 coupled between line 51 and servo motor 52. A feedbacknetwork or sensor 91 is connected in a conventional manner with summingoutput amplifier 90 in order to produce a servomechanism which seeksnull. As the signal on line 51 goes, for example, in a positivedirection, servo motor 52 is driven until the feedback signal fromnetwork 91 exactly balances the signal on line 51.

As the temperature and other extraneous influences on the structuresurrounding the mold 14 varies, the inductance of coils U and L canchange over a substantial range. The inductance changes cause aresulting change in frequency of oscillator 50, which causes the curvesof FIG. 4 to shift upwardly or downwardly from their illustratedposition. However, the difference in the frequency of oscillationbetween the pair of coils remains in the same proportion as thatillustrated in FIG. 4. Since the circuit of FIG. 3 is only responsive tothe difference rather than the absolute value of the frequencies, commonmode extraneous influences are eliminated. A common mode influence isone which affects both of the coils U and L equally, the describeddifferential type circuit therefore exhibits common mode rejection, inthat it discriminates against changes in a fundamental quantityaffecting both coils.

Although the circuit rejects common mode inductance and like effects, aninductance change could produce frequency changes of sufficientmagnitude so as to exceed the range of the fixed timing signalsgenerated by the circuit. In order to obviate such a problem, theoutputs of both of the pulse width detectors 83 are coupled to anaveraging network 95 which generates an output signal representative ofthe absolute sum of both pulse width detectors. This signal drives aconventional automatic frequency control (AFC) amplifier 96 to controlthe frequency of beat oscillator 64 in a known manner, as by varying thecapacitance in oscillator 64 in inverse proportion to the amplitude ofthe control signal from averaging network 95.

As the inductance of both coils decreases and the AC. resistanceincreases, for example, causing a resultant increase in the frequencyoutput of oscillator 50, the output of the pulse width detectors 83decreases. This produces a lesser amount of control signals fromaveraging network 95. The AFC circuit 96 is responsive to a lesseramount of control signal to increase the frequency of beat oscillator64, returning the beat between the oscillators 50 and 64 closer to theoriginal value, and thus insuring that the timing signals produced bythe circuit have the proper time relationship to enable sample gates and81 at the proper time.

DETAILED CIRCUIT In the following detailed description of the schematicdiagram, the folowing conventions will be used. All flipflops (FF) haveeither a single output line, or a pair of output lines labeled A and B.The B output is at all times opposite the A output. All signals areeither at a zero voltage level or at a positive voltage level withrespect to zero volts.

The legends carried by the inputs to the flip-flops indicate the mannerin which the flip-flop is triggered by an input signal. An input labeledRUN indicates that an input pulse will change or flip the instantaneousoutput of the flip-flop, regardless of the particular polarity values ona single or pair of output lines. An input labeled A means that an inputpulse will produce a positive output pulse or waveform at the A outputof the flip-flop. Conversely, an input labeled A means a positive pulsewill produce a negative pulse or waveform (i.e. a zero signal) at the Aoutput of the flip-flop. All flip-flops trigger on the positive goingedge of an input waveform.

While separate blocks are illustrated for each function the circuit isto perform, it should be understood that, in some cases, more than onefunction may be performed by the same component, and that the operationof a particular function block may be performed by a single resistor orother passive or active electrical component combined with othercomponents performing the func tions of other blocks in the circuit.

Turning now to the detailed drawings, the structure and operation of thecircuit of FIG. 5 will be described in connection with the waveformdiagrams illustrated in FIG. 6, which show the time relation between thevarious signals generated by the circuit. Throughout the specification,values will be given for certain frequencies and certain time relationsbetween signals in order to disclose a complete, operative embodiment ofthe invention. However, it should be understood that such values aremerely representative and are not critical unless specifically sostated.

Coil oscillator 50, FIG. 5A, may be a Colpitts circuit having componentvalues so that when either of the coils U or L is switched into theoscillator circuit, a natural frequency of oscillations on the order of200 kilocycles (kc.) or so is generated. The coil electricalcharacteristics, especially inductance and AC. resistane, are believedto be mainly affected by the periphery and surface effects of the moltenmetal, with little penetration of the oscillations into the actualcenter of the molten metal. The nominal frequency of oscillation ischosen by considering several factors. The number of cycles change orshift due to a level change is greater for higher frequencies. However,lower frequencies provide better penetration of the mold and castingmachine.

For one particular embodiment, coil U was formed from 30 turns of wire;while coil L was wound in two sections, the upper section L of FIG. 2being formed from 18 turns of wire, and the lower section L" beingformed from 19 turns of wire. Although it was found advantageous tosplit coil L into two portions, such a construction is not essential.The splitting of coil L is be lieved, for certain continuous castingmachines, to aid in averaging out the thermal effects across the coilsand in distributing the effect of the large masses of metal of thecontinuous casting machine, which acts as a shorted secondary and tendsto desensitize the coils.

For any given casting machine, the volume within which the coils can bemounted is of certain practical limits, and it is desired that a minimumnumber of turns be used within this volume. Lesser turns allows aproportionate increase in wire size, lowering the resistance of the coilin order to incerase the Q of the circuit.

Since molten metals tend to approach generally the same resistivity,although the resistivity of the metals in the solid state may besubstantially different, the level detector is generally insensitive tothe type of metal being cast.

As electronic switch 60 alternately connects coils U and L to oscillator50, a train of output oscillations is generated on line 61, with aparticular portion 100, FIG. A, of the oscillations being attributableto coil U, and the remaining portion 101 of the oscillations beingattributable to coil L. This multiplex operation, in which each coiltime shares a common oscillator, prevents the coils from interacting. Asthe metal level varies several inches from its desired value, thefrequency of oscillations from oscillator 50 will vary about 1,000c.p.s. or so from the nominal 200 kc. center frequency.

Beat oscillator 64 produces a continuous frequency output at 194.180kc., which beats or heterodynes in detector 63 with the oscillationsfrom oscillator 50 produce a difference or beat frequency of nominally5.820 kc. Wave shapers 105 and 106 are connected to the output of coiloscillator 50 and beat oscillator 64, respectively, to eliminateextraneous signals. Each wave shaper 105 and 106 may have a diode gatepoled to pass only positive portions of the waveform to mixer-detector63. The resulting beat frequency signal at output 65 is coupled to awave shaper 108, in the form of a Schmitt trigger, in order to produce asquare wave output of nominally 5 .820 kc., as illustrated in FIG. 6T.

The sample rate at which coils U and L are alternately switched orconnected with oscillator 50 is determined by the control requirementsof the continuous casting process, and depends upon the casting rate forthe machine and the response time of the system. For a particularmachine and a casting rate on the order of 100 feet per minute, thesample rate was chosen to be approximately 400 cycles per second. Forsuch a sampling rate, clock 70 has a repetition rate for its completecycle of 1,250 microseconds. As seen in FIG. 5A, the output of clock 70is nonsymmetrical, and consists of a first portion on line A having aduration of 250 microseconds, and a subsequent portion on line B havinga duration of 1,000 microseconds.

For simplification, only one complete cycle of clock 70 is shown in FIG.6. During each 1,250 microseconds, only one of the coils is sampled,which for illustrative purposes is coil U (FIG. 6B). It should beunderstood that during the next complete cycle of clock 70 (notillustrated), the opposite coil L would be connected to coil oscillator50, and would produce the same relative time relation between thesignals as illustrated for coil U.

Sample gates 80 and 81 are NAND logic blocks, activated by the absenceof all positive inputs, i.e., all inputs must be negative in order toproduce a positive output. The square wave output of wave shaper 108 isinverted by a detector gate 112, FIG. 5A, when actuated or enabled bythe positive output of lock-out bi-stable switch 114. The two inputs toswitch 114 are coupled to the A and B outputs of clock 70 (and to othercircuitry, described later, for blocking switch 114, causing switch 114to have an output at A after the lapse of the first 250 microsecondsfrom the start of each cycle output from clock 70, FIG. 6B. This outputopens detector gate 112, causing it to invert the output of wave shaper108, FIG. 6T, and thus causing a series of three pulses, FIG. 6F, to bepassed to the sample gate inputs.

Another input to each sample gate 80 and 81, FIG. 5B, is coupled to adifferent A or B output of steering FF 72, FIG. 5A. Since steering FF 72only triggers on a positive going waveform, it in effect sums thenonsymmetrical output of clock 70, producing an output waveform having atotal cycle time of 1,250 microseconds. When the A output of steering PF72, FIG. 6D, enables switch coil 60 for coil U, thus causingoscillations 100, FIG. 5A, attributable to coil U, the same positive Aoutput, coupled to sample gate 81, disables the L sample gate. The Boutput of steering FF 72, which is negative, is coupled to sample gate80, and thus tends to enable the sample gate for the U coil.

Timing circuit produces timing signals which allow only the latterportion of the third cycle of oscillation which exists beyond a fixedtime period to pass to the pulse width detectors 83. When each coil isfirst connected to coil oscillator 50, a small transition time occursbefore the oscillatory output on 61 is actually representative of thenatural frequency of oscillation of the circuit. The initial 250microsecond output A from clock 70, coupled to lock-out bi-stable switch114, prevents the initial oscillations from being counted or tootherwise determine the third pulse which will be passed by the samplegates.

When lock-out switch 114 enables detector gate 112, the first positivegoing output from the detector gate, FIG. 6F, is coupled to the RUNinput of a sample R-S flip-flop 120, FIG. 5B, triggering the flip-flopout of its reset condition to initiate the start of the count which willcause only the back portion of the third pulse to be gated throughsample gate 80. It should be noted that sample R-S flip-flop waspreviously triggered to a reset condition by input A, coupled to the Aoutput of clock 70.

Sample R-S flip-flop 120, being triggered only by the positive goingedges of the pulse outputs from detector gate 112, in effect divides bytwo, as can be seen in FIG. 6G. The A output from FF 120 is connected toan input of each of the sample gates 80 and 81. The B output of FF 120triggers the RUN input of a sample auxiliary (AUX.) R-S flip-flop 122,FIG. 5B, which again divides by two, and couples its A output to aninput of each of the sample gates 80 and 81. The sample AUX. FF 122 wasinitially reset by the A output of clock 70 which is coupled to an Ainput of FF 122.

The B output of sample FF 120 is also coupled to a delay timer 124, FIG.5B, which when actuated is delayed in resetting by a fixed amount oftime, as 400 microseconds, FIG. 6]. The time interval is chosendepending upon the frequencies and time durations of the signalsgenerated by the circuit, and as can be seen by referring to FIGS. 6]and 6T, the time interval is chosen to lapse at some point during theoccurrence of the third pulse from wave shaper 108.

The A output from delay timer 124 is coupled to a reset timer 126, FIG.5B, which resets for a fixed time period, as 10 microseconds, when the Ainput thereto goes positive, as can be seen by comparing FIGS. 6] and6K. The purpose of reset timer 126, as will appear hereinafter, is toproduce a short timing pulse at output B which is used to discharge thepulse width detectors 83. The A output of reset timer 126 is coupled toa gate bi-stable switch 128, FIG. 5B, producing an output at A which isalso coupled to sample gates "80 and 81. As can be seen from FIG. 6L,the A output of switch 128 goes negative as the A output of reset timer126 again goes positive.

At this time, all inputs to sample gate 80 are negative, and,accordingly, sample gate 80 produces a positive output sample pulsehaving a duration equal to the remaining width of the pulse from waveshaper 108 (which is inverted by detector gate 112 to produce the signalseen in FIG. 6F). The sample pulse output, FIG. 6M, from sample gate 80has a width which is proportional to the duration of the portion of thethird cycle oscillation which occurs after a predetermined point fixedwith reference to time. As the frequency of oscillation varies, causedby a At the end of the third cycle of oscillation, the square waveoutput of wave shaper 108, FIG. 6T, drops to Zero, causing the invertedoutput from detector gate 112 to rise in a positive manner, as seen inFIG. 6F. This positive output from detector gate 112, which is coupledto both sample gates, now blocks sample gate 80 and thus ter rninatesthe end of the reading of the sample.

The positive output from detector gate 112 is also coupled to the RUNinput of sample R-S PF 120, and the A input .of gate bi-stable switch128. This causes PF 120 and switch 128 to reset to their positiveconditions, as seen in FIGS. 6G and 6L, respectively. The resulting Aoutput of gate bi-stable switch 128 is coupled to the A input oflock-out bi-stable switch 114, thereby terminating the A output ofswitch 114, as seen in FIG. 6E. As the A output of switch 114 returns tothe zero level, it blocks detector gate 112 from passing any furthersignals from wave shaper 108. This returns the various flipflops intiming circuit 85 to their rest or reset condition until the start ofthe next 1,250 microsecond output from clock 70, which now switchesflip-flop 72 to initiate the above described series of operations allover again, but this time for coil L.

Each of the pulse width detectors 83 preferably has a rapid rate ofresponse, in order that the system may follow rapidly changing levels ofmolten metal within the mold. Any conventional circuit may be used fordetectors 83 which produces an analog voltage output proportional to thewidth of a pulse input, and which can respond quickly to pulse inputshaving varying widths. An integration circuit, by itself, does not havea sufficiently short time constant to meet these requirements.

By way of example, detectors 83 may consist of a dischargeableintegration circuit which charges during the occurrence of a samplepulse, and holds or maintains the voltage to which the integrationcircuit has been charged after the cessation of the sample pulse. Theheld or maintained charge may then be discharged just prior to theoccurrence of another sample pulse by a reset circuit which shorts orjumpers the integration circuit.

For this purpose, a pair of reset switches 130 are associated with eachof the pulse width detectors 83. Each reset switch 130 is coupled to adifferent output from steering flip-flop 72, in order that only thereset switch 130 corresponding to the coil which is being read at thattime will be enabled. The other input to the reset switches is coupledto the B output of reset timer 126, and is fed the microsecond pulse,FIG. 6K, which occurs immediately prior to the time at which the samplepulse will be gated through the sample gate.

During the simultaneous occurrence of the positive discharge pulse fromreset timer 126 and a positive output from steering flip-flop 72, thereset switch 130 corresponding to the coil being switched to the coiloscillator generates a positive output which is used, by means of anyconventional circuit, to discharge the pulse width detector associatedtherewith. It should be noted that the inherent time delay necessary forthe pulse from the A output of timer 126 to be coupled through gatebistable switch 128 and enable the sample gate, is sufficient to allowthe pulse width detector 83 to fully discharge and reset prior to theoccurrence of the next sample pulse. Thus, the pulse width detectors 83maintain an analog voltage output which exists throughout the readingcycle, except for the short 10 microsecond discharge time.

Pulse width detectors 83 are coupled to differential amplifier 87, whichwith summing output amplifier 90 form a conventional 3-term control.More particularly, a differential amplifier 87 is formed from an outputdifferential amplifier 135, an integrate differential amplifier 136, anda derivative differential amplifier 137. The outputs from integratedifferential amplifier 136 and derivative differential amplifier 137 donot cancel since the time of response of each is substantiallydifferent. The derivative difi'erential amplifier 137 responds to therate of change 12 of the analog signal, and has a time constant of aboutsecond, whereas the integrate differential amplifier 136 has a timeconstant in the range of 20 seconds, and responds roughly in proportionto the product of the input analog signal and time.

In order to smooth the fluctuations which would otherwise occur when thepulse width analog signal is terminated during the time that the pulsewidth detectors 83 are being discharged, a DC level compensationamplifier 139 responds in inverse proportion to the input to outputvoltage swing of output differential amplifier 135, in order to maintainan average sum level to the remaining differential amplifiers.

The servomechanism may use any conventional circuit, and as illustrateduses a servopower amplifier 141 which is responsive to the output fromsumming output amplifier to drive servomotor 52 in the proper direction.The direction is determined by which signal at the two inputs toamplifier 141 exceeds the other signal.

While the circuit of FIG. 5 has been described in detail, it will beapparent that other systems could be used in accordance with theteachings of this invention. With in the purview of this invention, itwill be apparent that whether the conductor whose level is being sensedis of relatively good or bad electrical conductivity is unimportant. Itis merely necessary that the mass of material whose level is beingsensed exhibit sufiicient conductivity to allow circulating eddycurrents to be induced therein, producing a loss which is reflected backto the coil generating the magnetic field which induced the eddycurrents. Thus, as used in the context of a level determining apparatus,the term conductor is defined to be any ma terial capable of producing achange in an electrical characteristic of a coil located adjacentthereto. Rather than using a change in an electrical characteristic ofthe coils to produce a change in frequency which is measured by thephase shift between oscillatory signals, it will be apparent that othersystems which accomplish the same purpose could be substituted therefor.Other changes and modifications apparent to those skilled in the art areintended to be within the scope of the invention, as defined in thefollowing claims.

I claim:

1. In a continuous casting machine including a mold into which moltenmetal is poured and means for varying the level of the molten metal insaid mold, a control for maintaining the molten metal at a predeterminedlevel in said mold, comprising:

coil means adjacent said mold and responsive to a change in level ofmolten metal within said mold for varying an electrical characteristicof said coil means;

means coupled to said coil means for producing an output signalproportional to said electrical characteristic of said coil means; and

means for controlling said level varying means to cause the level ofsaid molten metal to be proportional to said output signal.

2. The control of claim 1 wherein said coil means comprises a pair ofcoils located adjacent said mold and responsive to a change in level ofmolten metal for varying at least the inductance of each coil, and

said signal producing means is responsive to the difference ininductance between said pair of coils for producing said output signal,said output signal being independent of a common mode change ininductance of said pair of coils.

3. The control of claim 2 wherein said coils have the same inductancewhen said molten metal is at said predetermined level.

4. The control of claim 2 wherein said signal producing means includesan oscillator having a self-resonant frequency dependent upon the valueof inductance of the coil coupled thereto, and means for alternatelycoupling said pair of coils to said single oscillator.

13 14 5. The control of claim 1 wherein said signal produc- FOREIGNPATENTS ing means includes oscillator means coupled to said coil 226 8944/1963 Austria means for generating oscillations having a frequency de-711,067 6/1965 Canada pendent on the electrical characteristic of saidcoil means, 1)029:O98 3/1953 France:

and means cou pled to said oscillator means for convert- 5 1,373,1464/1964 France. ing said oscillations into said output signal. P

References Cited J. SPENCER OVERHOLSER, Primary Examiner UNITED STATESPATENTS R. S. ANNEAR, Assistant Examiner 2,905,989 9/1959 Black 16415510 us. c1. X.R. 3,204,460 9/1965 Milnes. 324-34 3,456,715 7/1969Freedman et a1. 164-155

